Characteristics of Serotonin Receptors in the Rat Brain

Two biochemical methods are currently available for studying 5-HT receptors in the central nervous system. The first consists of measuring the specific high affinity binding of 3H-5-HT to synaptic membranes. The other derives from the discovery of an adenylate cyclase which can be activated by 5-HT in brain homogenates. Whereas the specific 3H-5-HT binding is measurable in young as well as in adult rats, the 5-HT-sensitive adenylate cyclase can be quantitatively estimated only during the first three weeks following birth. Later on, the increment of adenylate cyclase activity produced by 5-HT is too low to permit valid measurements, notably in tissues from adult rats. Studies on the effects of various agonists and antagonists demonstrated that the specific binding site characterized by a high affinity for 3H-5-HT (Kd = 1.5 nM) exhibited the expected properties of a 5-HT receptor in brain. Performing chemical lesions on serotoninergic neurons by an intracerebral injection of 5, 7-dihydroxytryptamine or the blockade of central 5-HT receptors by the peripheral administration of methiothepin resulted in a subsequent increase in the number of specific binding sites for 3H-5-HT particularly in the hippocampus (+30 to +45%). In contrast, preliminary attempts to detect any supersensitivity of the 5-HT-sensitive adenylate cyclase after selective raphe lesions were unsuccessful. Indeed, several observations strongly suggested that the high affinity binding site for 3H-5-HT did not correspond to the 5-HT receptor coupled to adenylate cyclase in synaptic membranes: 1) the apparent affinity of the 5-HT-sensitive adenylate cyclase for 5-HT was about 300 times lower (Kd = 0.5 microM) than that of the specific 3H-5-HT binding site; 2)the ontogenic evolutions of 3H-5-HT binding and 5-HT-sensitive adenylate cyclase were not parallel, notably in the hippocampus; 3) they were differently affected by several drugs. For instance, quipazine, a putative 5-HT agonist, effectively displaced 3H-5-HT from its specific binding site (Ki = 0.23 microM) whereas it did not affect 5-HT-sensitive adenylate cyclase. In conclusion, it is likely that the high affinity binding site for 3H-5-HT and the 5-HT-sensitive adenylate cyclase belong to two different postsynaptic 5-HT receptors in the rat brain.